Wells are generally drilled into the ground or ocean bed to recover natural deposits of oil and gas, as well as other desirable materials that are trapped in geological formations in the Earth's crust. Wells are typically drilled using a drill bit attached to the lower end of a drill string. Drilling fluid, or mud, is typically pumped down through the drill string to the drill bit. The drilling fluid lubricates and cools the drill bit, and may additionally carry drill cuttings from the borehole back to the surface.
Reservoir production and testing may involve drilling wells and monitoring various subsurface formation parameters. When drilling and monitoring, downhole tools having electric, mechanical, and/or hydraulic powered devices may be used. In some implementations, pump systems may be used to draw and pump formation fluid from subsurface formations. A downhole string (e.g., a drill string, coiled tubing, slickline, wireline, etc.) may include one or more pump systems depending on the operations to be performed using the downhole string, or the string may have fluids pumped therein from a surface of the formation.
One such pump system is a positive displacement pump. A positive displacement pump causes a fluid to move by trapping a fixed amount of it then forcing (displacing) that trapped volume through a discharge. Such a pump system usually produces the same flow at a given speed (RPM) regardless of the discharge pressure.
Commonly, multiple moving parts involved in any formation testing tool, such as pump systems in either wireline or measurement-while-drilling (MWD) tools, can result in equipment failure or less than optimal performance. Further, at significant depths, substantial hydrostatic pressure and high temperatures are experienced, thereby further complicating matters. Still further, formation testing tools are operated under a wide variety of conditions and parameters that are related to both the formation and the drilling conditions.